dmaengine: dw-edma: Convert debugfs descs to being heap-allocated

Currently DW eDMA debugfs node descriptors are allocated on the stack,
which won't work for multi-eDMA platforms. As a preparation to supporting
multi-eDMA systems, allocate each debugfs node separately.  Afterwards
we'll add info like Read/Write channel flag, channel ID, DW eDMA private
data reference.

Note: this conversion is mainly required due to having the legacy DW eDMA
controllers with indirect Read/Write channels context CSRs access. If we
didn't need to synchronize access to these registers, the debugfs code of
the driver would have been much simpler.

Link: https://lore.kernel.org/r/20230113171409.30470-14-Sergey.Semin@baikalelectronics.ru


Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Vinod Koul <vkoul@kernel.org>